The key system parameters that affect the performance of a digital transmission system in long haul Dense Wavelength Division Multiplex (DWDM) transmission systems are fiber dispersion, optical noise and optical fiber nonlinearity. Long haul systems typically span over 1000 km. Fiber dispersion distorts transmitted pulses and increases the probability of errors in the digital transmission. Also, the optical signal is amplified periodically along the fiber (typically every 80 km), generally using an erbium doped fiber amplifier (EDFA), to compensate for loss in the transmission fiber. This amplification is accompanied by degradation in the optical signal-to-noise ratio (OSNR) of the signal, which also increases the probability of errors. This degradation can be partly mitigated by increasing the signal power launched into the fiber. However, increasing launch power tends to increase optical fiber nonlinearities, which in turn degrade the signal. The fiber nonlinear effects include self phase modulation (SPM), cross phase modulation (XPM), four waves mixing (FWM), stimulated Brillion scattering (SBS) and stimulated Raman scattering (SRS), all of which are well known in the art.
In a long haul DWDM transmission system, the digital signal is typically intensity modulated: the 1 bits have substantially higher power than the 0 bits. The ratio of power in the 1 bits to the power in the 0 bits is called the extinction ratio, and is typically 10-13 dB. Since nonlinear effects scale with instantaneous optical power, as opposed to average power, the 1 bits and 0 bits have different nonlinear effects. Also, for a non-return-to-zero (NRZ) coding, a sequence of 1s (e.g., 0110) has a lower peak power than an isolated 1 bit (e.g., 010). Therefore, different patterns experience different distortions in the presence of nonlinearity and fiber dispersion. Higher optical dispersion exacerbates the effect of nonlinearity. Furthermore, nonlinear distortion cannot be compensated for by dispersion-compensating fiber.
Several methods have been proposed to improve dispersion tolerance of transmission in the presence of optical nonlinearity: the use of distributed Raman amplifiers, the use of a return-to-zero (RZ) transmission format, the use of a differential phase shift keying (DPSK) format, the use of an optical duobinary format, the use of electronic dispersion compensation (EDC), etc.
Up to now, no one method has adequately addressed the foregoing optical transmission issues without significantly complicating system design. Also, some of the above-identified solutions require a carefully designed dispersion map. This means that dispersion-compensating fibers need to be installed with appropriately designed values of dispersion along the length of the transmission system so as to generate the desired “dispersion map” for best performance.
The DPSK format can improve system tolerance to nonlinear effects, but with little improvement of dispersion tolerance. With distributed Raman amplifiers, WDM system can also improve dispersion tolerance, but does not improve nonlinear tolerance. An optical duobinary format has +/−3000 ps/nm of dispersion tolerance, but is not tolerant to dispersion in the presence of nonlinearity. As is known in the art, electronic dispersion compensation (EDC) can only increase system performance marginally due to the nonlinearity in the detector. The RZ format allows higher launch power and increases OSNR but at the cost of reducing dispersion tolerance.
It is, therefore, an object of the present invention to increase the dispersion tolerance of a transmission system in the presence of optical nonlinearity.